1. Field of the Invention
The field of the invention is semiconductor processing. Specifically, the invention relates to incorporation of an impurity into a semiconductor film.
2. Background of the Invention
Incorporation of an impurity (such as carbon) into semiconductor films has proved beneficial in enhancing the electrical characteristics of devices such as heterojunction bipolar transistors and solar cells. In general, such improvements are achieved by modifying the mobility of carriers or the bandgap of the semiconductor material.
A low pressure chemical vapor deposition (LPCVD) in a single wafer or a multi-wafer deposition tool can be used to form an epitaxial silicon or polysilicon film. In order to introduce an impurity during the silicon growth process, a conventional polysilicon LPCVD process in a multi-wafer deposition tool utilizes a carrier gas to dilute an impurity-containing gas in order to achieve a desired concentration of impurity in the silicon film. Alternatively, a small flow controller can be used to control the flow of the impurity-containing gas into the deposition tool without a carrier gas.
As semiconductor devices decrease in size and requirements to improve device performance increase, lower concentrations of impurities in epitaxial or polycrystalline silicon films are required. For the carrier gas method, increasing the amount of the carrier gas to further dilute the impurity-containing gas adversely affects the polysilicon deposition process by modifying the reaction thermodynamics and kinetics. Moreover, dilution of the impurity-containing gas decreases the partial pressure of the reactant gas, thus slowing down the reaction to form the polysilicon film resulting in an increase in process time. Still further, the reduction in the growth rate of the polysilicon film can adversely affect the desired grain structure of the polysilicon film.
Another option is to simply decrease the flow of the impurity gas. However, at such low concentrations it is very difficult to control the amount of impurity incorporated into the silicon film. In addition, both the carrier gas and the decreased impurity gas flow methods involve delivery of the impurity-containing gas from an impurity-containing gas source external to the deposition tool, which increases the complexity of the deposition tool.
Accordingly, a need has developed in the art for a process of incorporation of an impurity in a thin film without adversely affecting the deposition process.
It is thus an object of the present invention to provide a method for incorporating an impurity in a thin film without adversely affecting the deposition process.
It is another object of the present invention to incorporate an impurity without increasing carrier gas flow or decreasing dopant gas flow.
The foregoing and other objects of the invention are realized, in a first aspect, by providing an impurity cell in a deposition chamber and desorbing impurity in a gas phase from the impurity cell into the deposition chamber, wherein the impurity is incorporated into the thin film.
In a second aspect, the invention is realized by providing an impurity cell in a deposition chamber and desorbing impurity in a gas phase from the impurity cell into the deposition chamber, wherein the impurity is incorporated into the thin film, and replenishing the impurity cell in the deposition chamber.